Various types of loads must be supported at extremely high elevations. High voltage power transmission lines and lighting systems are typical of such loads. The extremely high towers were first used in connection with transmission lines, and originally lattice type towers were used, and these towers have been rather satisfactory from a structural standpoint, but are aesthetically unsightly. Tubular towers made of steel plate material have been used in substitution for the lattice type towers, both for supporting transmission lines and lighting systems. The tubular towers provided some advantages structurally as compared to the lattice towers, and have provided substantial improvement over the lattice towers from an aesthetic standpoint; and the tubular towers have been wholly satisfactory from the strength and structural standpoint.
As it has been necessary and desirable to support transmission line and lighting system loads at higher and higher locations, the towers must be accordingly higher and higher. Some of the aesthetic advantages of the tubular steel towers is lost as the height of the towers increases because the base areas of the tubular towers must increase to substantial dimensions, and it has been experienced in the recent past that towers of approximately 180 feet in height must be approximately 9 feet across at the base.
One of the critical design requirements in tubular towers is the magnitude of tension under conditions of maximum wind loading tending to bend or bow the tower transversely from its normal perfectly upright position. Of course, the structural steel plate material which is used in fabrication of the tower must be able to withstand the maximum tension loads thereby applied.
It should be understood that when a tubular tower is wind loaded, and the tower is flexed in response to the pressure of the wind, the sheet steel in the windward side of the tower is under tension, and the sheet steel in the leeward side of the tower is under compression; and the sheet steel in the tubular tower, at locations along a plane facing windward and substantially extending through the tower axis, has shear stresses applied, these being the locations where the tensile forces change to compressive forces in the sheet steel. In general, it is considered that approximately half the sheet steel in the tubular tower is under tension and half under compression. When the bowing or bending of the tower causes deformation of the sheet steel, particularly where the sheet steel is under tension, beyond the elastic limits of the material, a permanent deformation is obtained, and the tower will no longer return to its desired exactly upright original position.
As a result of this possibility of permanent deformation, the tower must be extremely strong and broad at its base so as to withstand the maximum wind loading.